1. Field of Technology
The present invention relates to a method for the fabrication of thin-film transistors employed as elements which are formed on a single-crystal substrate, for example, field-effect transistors, thin-film transistors formed on an insulator, logic circuits formed therefrom, and a structural component of display drive circuits or display pixels used in electronic apparatuses such as memory circuits, liquid-crystal displays, and organic EL displays.
2. Description of the Related Art
Semiconductor films such as polycrystalline silicon films have been widely used in thin-film transistors (abbreviated as TFT hereinbelow) or solar cells. In particular, polycrystalline silicon (poly-Si) TFT can be produced on a transparent insulating substrate such as glass substrate, while allowing for increased mobility. Polycrystalline silicon TFTs have been widely used as light modulating elements, for example, in liquid-crystal displays (LCD) or liquid-crystal projectors or as structural elements of internal drivers for liquid-crystal drive and have successfully emerged into new fields of application, demonstrating there the above-described merits. A fabrication method termed as a high-temperature process has already found application for the manufacture of high-performance TFT on a glass substrate. The high-temperature process is one of the processes for TFT fabrication, which uses a high temperature of about 1000° C. The high-temperature process has advantages of: allowing for the production of polycrystalline silicon of comparatively good quality by solid-phase growth of silicon; and allowing for the formation of a good gate insulating film (typically silicon dioxide) and a clean polycrystalline silicon—gate insulating film interface by thermal oxidation. Those advantages of the high-temperature process make it possible to fabricate a high-performance TFT having high mobility and reliability with good stability.
However, for a substrate where a TFT is produced to be suitable for the high-temperature process it has to withstand high-temperature processing at a temperature of no less than 1000° C. Presently quartz glass is an appropriate material for transparent substrates satisfying this requirement. For this reason, the conventional polycrystalline silicon TFT have been fabricated on small and expensive quartz glass substrates, this process being cost ineffective and unsuitable for the transition to large-scale structures. Furthermore, since the solid-phase growth method requires long-term heat treatment (several tens of hours), the productivity is typically very low. Another problem associated with this method is that the substrate undergoes large thermal deformations because the entire substrate is subjected to long-term heating. As a result, inexpensive large glass substrates could not be used. This factor also inhibited cost reduction.
By contrast, the technology referred to as a low-temperature process resolves the above-described drawbacks inherent to the high-temperature process and allows for fabrication a high-mobility polycrystalline silicon TFT. The low-temperature process is a process for the fabrication of a polycrystalline silicon TFT in which in order to use comparatively inexpensive heat-resistance glass substrates, the maximum process temperature is set to no less than about 600° C. Laser crystallization technology in which the crystallization of a silicon film is conducted by using a pulse laser with a very short generation time has been widely used in the low-temperature process. Laser crystallization is a technology employing the capability of an amorphous silicon film located on a glass substrate to be crystallized in a process of solidification after being instantaneously melted by irradiation with a high-power pulse laser beam. In recent years a technology of forming a large-area polycrystalline film by scanning an amorphous silicon film located on a glass substrate with an excimer laser beam, while repeatedly irradiating the film, has found wide application. Furthermore, a silicon dioxide (SiO2) film can be formed by a film forming method using plasma CVD and the prospects for practical application of this method for growing gate insulating films are promising. The above-described methods made it possible to fabricate a polycrystalline silicon TFT on a large substrate of several tens of centimeters on a side.
However, a problem associated with the low-temperature process is that high-density interface levels appear on the interface between the semiconductor surface serving as an active layer and a gate insulating film (referred to as MOS interface hereinbelow), significantly affecting mobility and the threshold voltage of TFT. The density of interface levels at a good MOS interface obtained by thermal oxidation at a temperature of no less than 1000° C. can be decreased to about 2×1010 cm−2eV−1, but when the insulating film was grown at a low temperature of no higher than 400° C. by plasma CVD and the like, the MOS interface level density was as high as 1011˜1012 cm−2eV−1. Since the energy of those interfaces levels is in the semiconductor band, the carriers can be easily trapped.
In case of a field-effect transistor, if a voltage is applied to a gate electrode, carriers defined by the capacitance of MOS capacitor are induced at the semiconductor side. However, if defects are present at the semiconductor side, that is, at MOS interface, the induced carriers are trapped by the defects and make no contribution to conductivity. As a result, unless a higher gate voltage is applied and more carriers than defects are induced, no drain current can be obtained. This is the reason for increasing the threshold voltage of TFT. At present, there are no effective means for reliably controlling the defects. As a result, the threshold voltage of TFT is high or a large spread is obtained between the lots, which rises serious problems associated with the currently employed manufacturing processes.
At present the threshold voltage of polycrystalline silicon TFT manufactured by the low-temperature process is about 3-4 V. If the threshold voltage is increased, for example, to about 1 V, the drive voltage of circuits fabricated by using the TFT can be reduced to less than one third of the present value. Since energy consumed by a circuit is proportional to a second power of the drive voltage, the decrease of the drive voltage to less than one third will make it possible to decrease the energy consumption dose to one tenth of the present value. Therefore, it will be possible to produce liquid-crystal displays with an ultralow energy consumption suitable for displays designed for portable information devices. To attain this object, the surface density of defects in both the poly-Si and the MOS interface should be decreased to about 1010 cm−2eV−1.